Optical amplifiers have been used extensively for transmission of optical signals in communication systems. Doped fiber amplifiers (DFAs) are optical amplifiers that use a doped optical fiber as a gain medium to amplify an optical signal. The signal to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with the doping ions. The most common example is the Erbium Doped Fiber Amplifier (EDFA), in which the core of a silica fiber is doped with trivalent erbium ions and can be efficiently pumped with a laser.
A lot of technological improvements have been made since the first EDFA was produced in the 1980s. Some components, however, are still required in each EDFA. For example, a Gain Equalizer filter is required to equalize channels. In addition, because the gain profile of the doped fiber is not equal for all frequency regions, a Variable Optical Attenuator (VOA) is required to implement a Variable Gain Amplifier (VGA). A side effect of the VGA is that there is only one gain point where the Noise Figure (NF) is optimized. Lower gain values have degraded NFs. For higher gains, the amplifier shows a “tilted” spectrum.